1. Field of the Invention
The invention relates generally to the field of dielectrics. More particularly, the invention relates to lossy dielectrics.
2. Discussion of the Related Art
Nearly all high power microwave sources used for radar and communications rely on vacuum electron devices (xe2x80x9ctubesxe2x80x9d) to create or amplify the microwave signal. A wide variety of these devices exists; oscillator tubes such as magnetrons and gyrotrons operate at a more or less fixed frequency, whereas amplifier tubes such as klystrons and traveling wave tubes (TWTs) may amplify signals over some frequency range. TWTs can be configured in a number of different design variants such as ring-loop, ring-bar, coupled-cavity, and helix TWTs. Those familiar with the design of microwave amplifier tubes have long appreciated that it is necessary to suppress various undesirable microwave signals that may arise during operation, because if these undesirable signals are not suppressed the tube may begin to oscillate or suffer other performance degradation. Such undesirable signals may include feedback (a reflected signal propagating back toward the low-power circuit), harmonics of the desired signal, noise, etc.
To suppress the aforementioned undesirable signals, microwave amplifier tubes such as klystrons and TWTs generally incorporate loads of one form or another. The load is made of a material having a high dielectric loss, and its size, shape, and location within the tube are chosen to best attenuate undesirable signals while not attenuating desired signals. One tube may contain a number of these loads in the form of small cylinders, rings, wedges, etc. affixed to the tube structure at selected locations.
It will be appreciated that the load attenuates microwave power by converting it to heat, which must in turn be removed by conduction through the metal vacuum envelope of the tube. For this reason, loads have generally been made of BeO or AlN with a dispersed phase of SiC, in which the high loss tangent of the SiC converts the microwave power to heat and the excellent thermal conductivity of the BeO or AlN allows this heat to be conducted to the tube wall which is, in turn, typically air- or liquid-cooled copper. Conventional load materials typically contain about 40% SiC dispersed uniformly throughout a BeO matrix. As discussed later, this uniform dispersion of power within a second phase leads to less than desirable thermal performance.
Increasingly, there are economic and regulatory pressures to eliminate the use of BeO because of its toxicity. What is needed, therefore, is a less toxic approach to providing lossy dielectrics.
One approach was to use a composite of AlN with a uniform dispersion of SiC particles (about 40 vol. % SiC). The composites exhibited good dielectric properties, but the thermal conductivity was too low (less than 50 W/mxc2x0 K) to withstand high heat loads encountered in broad microwave applications.
In ceramics, heat conduction is dominated at moderate temperatures (less than about 600xc2x0 C.) by phonons (lattice vibrations). High conductivities are achieved in the least cluttered structures. In other words simple crystal structures with few kinds of elements, where the constituent atoms are similar in atomic weight, and the atomic bonding is high. AlN and SiC meet these conditions as single crystals and intrinsically possess high thermal conductivities. In polycrystalline materials, the thermal conductivity is directly proportional to the amount of scattering of the phonons (sometimes referred to as the mean free path) by defects. Defects are associated with secondary atoms in the crystal lattice, pores, grain boundaries and second phase particles. In a two-phase composite of AlN and SiC, scattering of phonons can occur at the interfaces between the two phases even though each phase exhibits high thermal conductivity at a single phase. In addition, because of differences in thermal expansion coefficients, microcracks or other incontinuities can develop between phases which would further decrease the thermal conductivity.
Thus, in a AlN-SiC composite with uniformly distributed phases as practiced in the prior art, phonon scattering occurs at the interfaces to significantly decrease the overall thermal conductivity. The layered composite structures described in the present patent are distinguished from the prior art by the presence of continuous high thermal conductivity paths between layers of high dielectric loss. The high thermal conductivity paths will dominate the overall thermal conductivity of the composite and result in a composite of high thermal conductivity.
Another disadvantage of the BeOxe2x80x94SiC approach has been relatively high cost. Therefore, what is also needed is a solution that meets the above-discussed requirements in a more cost effective manner.
Heretofore, the requirements of environmental friendliness and economy referred to above have not been fully met. What is needed is a solution that simultaneously addresses both of these requirements. The invention is directed to meeting these requirements, among others.
A goal of the invention is to simultaneously satisfy the above-discussed requirements of environmental friendliness and economy which, in the case of the prior art, are mutually contradicting and are not simultaneously satisfied.
One embodiment of the invention is based on a multilayered configuration of high dielectric loss and high thermal conductivity-electrically insulating materials that is used to form a lossy dielectric. Another embodiment of the invention is based on a method wherein a continuous high thermal conductivity-electrically insulating material (e.g. aluminum nitride, magnesium oxide or silicon nitride) and a high dielectric loss (e.g. ceramic, metal, etc.) material are laminated to form a lossy dielectric.
The lossy dielectric can be a laminated composite that is optimally bonded to a thermal sink. The bonding can be effected by typical methods, such as brazing, soldering or mechanical shrink fitting. The laminated composite with the high thermal conductivity material on the exterior surfaces is preferably not electrically conductive.
These, and other, goals and embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description, examples and the accompanying drawings and the examples. It should be understood, however, that the following description, while indicating preferred embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such modifications.
Laminated multilayer composite structures are well known in the prior art for use as electronic devices for applications such as integrated circuit (IC) devices. For IC devices, the materials are typically Al2O3 or AlN and a conductor, such as Mo or W. In these prior applications, the devices are electrical conductors. Devices that embody the invention are not electrical conductors due to the fact that the high thermal conductivity material, which is continuous and on the exterior surfaces, is an electrical insulator.